Amine phosphate coating solutions and method of coating



E PHOSPHATE CQATING SOLUTIONS AND IVETHGD OF COATING William S. Russell, Royal Oak, Mich., assignor to Iarker Rust Pro-of Company, Detroit, Mich, a corporation of Michigan No Drawing. Application March 26, 1952, Serial No. 278,743

11 Claims. (Cl. 148--6.15)

This invention concerns the treatment of surfaces of mild steel, zinc, and aluminum for the purpose of producing thereon a protective coating without the necessity of pretreatment of the metal surface. The invention concerns particularly the coating of steel, zinc, and aluminum by the chemical action of a phosphate containing bath on the metal surface wherein the phosphate is an amine phosphate.

This application is a continuation-in-part of my prior copending application, Serial No. 209,323, filed February 3, 1951 for Amine Phosphate Coating Solutions, now abandoned.

It is common practice in the art of metal coating to treat metal surfaces with acidic solutions of phosphates where the phosphate may be phosphoric acid or a salt of phosphoric acid. It is also known that in particular instances an organic phosphate may be used in a nonaqueous solution and in other instances may be used to provide a source of phosphoric acid by the decomposition of the organic phosphate at an elevated temperature. in connection with organic phosphates for example, Prutton in U. S. Patent No. 2,224,695 discloses that esters and ester salt derivatives of phosphoric acid may be used to form a coating of an organic iron phosphate on the surface of a metal where the ester is used in a non-aqueous media. Further, he discloses that the presence of moi-sture accelerates the reaction of such organic phosphates in the formation of an organic iron phosphate coating.

An essential feature of the present invention lies in the use of an aqueous medium and the formation of a metal phosphate coating rather than an organic metal phosphate coating.

Phosphates have been classified as coating phosphates or non-coating phosphates depending on whether the cation associated with the phosphate becomes an essential constituent of the coating or whether the phosphate formed is substantially that of the metal coated. The difference between these two coating mechanisms can become very important from an economic point of view when the cation associated with the phosphate is derived from a relatively expensive source. Again using the above-mentioned patent by way of example, the coating formed is an organic iron phosphate. Therefore, the phosphate was used as a coating phosphate and the concentration of organic material would be depleted constantly during use. It is a feature of the present invention that an organic phosphate is used as a non-coating phosphate.

An additional serious problem which is encountered in this art concerns the nature of the metal surface to be treated. The eifectiveness of the best coating baths known is impaired considerably when the metal surfaces are dirty or contain finger marks, grease and other residues not normally soluble in the coating bath. This problem has been solved to some extent by pretreatment of the metal surfaces with grease solvents, sand blasting,

States atom etc. In general, however, the effectiveness of coating baths has been linked closely with the effectiveness of metal pretreatment.

It is, therefore, an object of this invention to provide a composition for the forming of an adherent metal phosphate coating on the surface of mild steel, zinc, or aluminum without requiring the extensive pretreatment of the metal surface now found necessary.

It is a further object of this invention to provide a preliminary cleaning coating for mild steel, zinc, or aluminum which can be readily converted to the standard phosphate coatings of the art or can be given a standard chromic rinse to form in both cases excellent paint bases without the necessity of the extensive pretreatment now required for the formation of these latter coatings.

It is also an object of this invention to provide for the formation of a metal phosphate coating in a onestep process; to provide for the formation of such a coating to be followed up by the use of a standard coating phosphate bath in a two-step process, or by a chromic rinse for a two-step process; or to provide for the forma tion of such a coating to be followed up by both the use of a standard coating phosphate bath and a chromic rinse for a three-step process where the one-, two-, and three-step processes produce a superior result in both cleaning and coating the metal and in providing excellent paint bases and drawing and wear-resistant surfaces.

It has now been found that the foregoing and related objects may be achieved with an aqueous coating bath which is maintained at a pH between 3 and 6 and which comprises as the principal coating-forming ingredient an amine phosphate which yields at least about .01 percent phosphoric acid in the coating bath. Improved quality coatings may be obtained by adding to such aqueous solutions, controlled proportions of certain oxidizing agents.

The amine phosphate to be used in the composition and the method of the present invention can be determined solely by its solubility in the composition; that is, all amine phosphates possess the property of dissolving to some extent in water in the pH range of 3 to 6, and their ionization yields at least some phosphate ions. The only consideration which must be satisfied is that the solubility of the amine phosphate be sufficient to provide a significant contribution to the phosphate ion concentration. Although this latter concentration is not critical, practical considerations cause me to include within the scope of my invention those amine phosphates which yield at least about .01 percent of a phosphate ion in the composition as a lower operating limit. A preferred range is .05 %.15% phosphate ion. There is no upper limit to the concentrations which may be used since all species are operable up to and including a satu rated solution. Of the large group of amine phosphates which satisfy the above requirement, a preferred group for the purposes of this invention are the amine phosphates having the following structure:

where R1 and R2 may each be a member of the class consisting of hydrogen and hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl; wherein R3 may be a mem ber of the class consisting of hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl; and wherein the foregoing hydrocarbon radicals may contain hydroxyl, bromo, chloro, sulfo and nitro substituent groups.

In aqueous solution, it will be understood that the amine phosphates of the above given structure are in actuality in the form of ions in equilibrium such that the substituted amine group mayoccupy any one of the three displaceab'le hydrogen positions of the phosphoric acid radical. When one of the hydrogens of .the phosphoric acid radical is substituted by one substituted amine group, the resulting aqueous solution has a pH-of approximately 4.2. When two of the displ-aceable hydrogens .of the phosphoric acid radical are substituted by amine groups, the pH of the aqueous solution approaches 7. As indicated hereina-bove, the operating range of pH of the solutions of this invention is restricted to between 3 and 6. It will be understood therefore that in order to achieve a pH of 3 in an aqueous solution of amine phosphates having the above given structure, it is necessary that there .be a slight excess of tree phosphoric acid in the solution. Solutions having a ,pH of about 6 contain a mixture of monoamine phosphates and diamine phosphates or, in :other words, the solution contains a .slight excess of amine over that requiredto replaceione hydrogen ion but less than is required to fully replace two hydrogen ions of each phosphoric acid radical.

An amine phosphate ionizes very rapidly in aqueous media to yield the equilibrium proportions vof the amine phosphate, phosphoric acid and amine. Similarly, phosphoric acid reacts rapidly inaqueous media with the same amine to yield the same equilibrium proportions. Therefore, the phosphate may be added as a mixture of the equivalent proportions of phosphoric acid and amine having the formula wherein R1 and R2 may each be a member of the class consisting of hydrogen and hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloa-lkyl; wherein R3 may be a member of the class consisting of hydrocarbon radicals having less than 7 carbon atomsand of the group consisting of alkyl, aryl and cycloa-lkyl; and wherein the foregoing hydrocarbon radicals may contain hydroxyl, bromo, chloro, sulfo and nitro substituent groups. In most instances where the pH of a'solution is to be corrected downward-1y, it is desirable to add phosphoric acid to serve the dual purpose of increasing the acidity and of increasing the concentration of phosphate .ions. Similarly, and where the pH is to be corrected upwardly, an alkali metal phosphate, carbonate, oxide, hydroxideor the like is most suitable. From the foregoing, it will be apparent that the composition of the solutions and the relative proportions of amines and phosphates which are contained therein are specifically dependent upon the recited pH.

Coatings of exceptionally good-quality may be obtained from the solutions of this invention when there is incorporated therein limited quantities of certain oxidizing agents. Although the presence of oxidizing agents .is not necessary in order to .achive the objects of the invention, particularly outstanding coatings result from the use of an oxidizing agent and this coating may be improved even further by the subsequent treatment. of the coating with a chromic rinse. Preferred oxidizing agents and the operating ranges of concentrations in the solutions of this invention are as fol-lows:

Other oxidizing agents which may be employed but which are somewhat inferior to the above preferred oxidizing agents for commercial reasons include sodiummetanitrobenzene sulfonate in concentrations .of 1 to 4 percent and .hydrogen peroxide :in 7 concentrations of ..0.01 percent to .05 percent. Within the above given ranges, thespecific percentage for each of the various oxidizing agents which is most suitable may vary considerably, but the optimum percentage is quite easily determined in each specific instance since the improved coatings resulting from the use of oxidizing agents are in each instance heavier coatings than are produced with the amine phosphate itself. This statement should not be interpreted as implying that there is any relationship between coating weight and quality of coating, for actually there is not. In this specific case, however, the improved quality coatings derived by the adding of an oxidizing agent are heavier-coatings and, thus, the coating weight may be taken as a reference standard to determine the proper percentage of oxidizing agent to yield an improvedqualitycoating.

The composition of the present invention may be painted or sprayed on the metal-surface or the metal may be immersed in a bath of the coating composition. The temperature of the solution and the time .ofcontact between metal and solution is not critical. Temperaturesof about 130'170 F. are most suitable forcoating metal in a relatively short time but temperatures down to room temperature can be .used. Visible coatings are formed on both steel and zinc but the coating on aluminum, although an excellent paint base, cannot be seen by visual inspection. The aluminum phosphate coating is'probably transparent because of its extreme thinness.

As indicated previously, an advantage is gained from the use of the compositions of this invention in those cases in which 'it is desired to form on the surface one of the well-known phosphate coatings such as is produced with a zinc phosphate solution; When zinc phosphate coatings or other metallic phosphate coatings produced from coating-phosphate solutions are desired, the zinc phosphate solution may be applied directly to the metal which has been preliminarily coated in accordance with the method of the present invention. In many instances the preliminarily formed coating will dissolve in the coatingphosphate bath and be replaced by the-phosphate coating from the second bath because the latter baths'are' treating the coatings with a hexavalent chromium rinse 'lent base for an additional or alternative coating.

' phosphoric acid were made up to 6 liters with water, givnormally run at quite high ac'idities. The improved coating which results from such a two-step method, appears to owe its quality to .the nature of the surface which resulted from the preliminary treatment in accordance with the method ofthis invention. 'That .is, the use of the composition of the present invention not only coats the metal but thoroughly cleans it and provides .an excel- The phosphate coatings produced by the method of this invention as well as phosphate coatings .of the conventional :zinc phosphate type resulting from .the .above described two-step method, may be further improved by in accordance with conventional methods and with 'chromic :acid concentrations which are new standard practice in this art.

The following examples are given to more clearly illustrate the method and the solutions of this invention:

Example 1 A series of solutions were made up as follows: a. 19.5 grams .isopropylamine and.26 ml. 75% 'phosphoric acid were made up to 6 liters 'with water, giving a'pH of 5.3; i

b. 19.2 grams monoethanolamine and 25.8 ml. 75%

ing a pH of 5.2; v

c. 30 grams cyclohexylarnine and 25 ml. 75% phosphoric acid were made up to 6 liters with Water, giving a pH of 4.95; r

d. 45.5 grams di-n-amylamine and 25 ml. 75% phosphoric acid were made up to 6 liters with water, giving a .pH of 4.5;

e. 56.5 grams tri-n-butylamine and 25 ml. 75% phos-1 phoric acid were made up to-6 liters with water, giving a f. A number'of solutions each containing 226 ml. of

triethanolamine were made up to 8 liters in water. To each of the solutions was added a different proportion of 75% phosphoric acid to produce a series of compositions having pH values in the range of 3.0 to 5.7.

Each of the preceding solutions at a temperature of 160 F. was sprayed on the surface of a cold rolled mild steel panel for a period of one minute, the steel panels having received no preliminary treatment. The coatings produced were very adherent and varied in color from grayish white to dark gray with an outside thin, light tan, dusty film. The coating weight varied from 15 to 100 mg. per square foot of surface area, and in each instance was essentially iron phosphate. The coatings were then rinsed with hexavalent chromium solutions in accordance with standard practice and the resulting coatings were found to be excellent paint bases.

Example 2 Each of the solutions described in Example 1 was modified by adding thereto varying proportions of chlorate, nitrite, bromate and sulfite oxidizing agents within the ranges of concentrations hereinabove indicated, and the processing carried out in the manner indicated under Example 1. Definite improvements were noted in each of the coatings, and the coating weights were in each case substantially greater than the corresponding examples without the oxidizing agent.

Example 3 Solutions having the compositions indicated under f of Example 1 were sprayed upon the surface of a cold rolled mild steel panel at a temperature of 160 F. for one minute. The treated panels were then subjected to spraying by a conventional zinc phosphate coating solution containing a nitrate accelerator at 160 F. for about one minute. The zinc phosphate coatings produced were rinsed with a standard hexavalent chromium solution. The coatings produced wereyfound to have excellent corrosion resistance, resistance to humidity and to be an especially satisfactory base for paint.

What is claimed is:

1. A method for cleaning and producing a metallic phosphate coating on a metal of the class consisting of mild steel, zinc and aluminum which comprises the step of contacting the surface of the metal with an acidic aqueous solution consisting essentially of phosphoric acid, water and a substituted amine phosphate having the structure:

R1 NR2 wherein R1 and R2 are each a member of the class consisting of hydrogen and hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, and R3 is a member of the class consisting of hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, said phosphate yielding-at least about .01% of a phosphate ion in the solution, the pH of the solution being between 3 and 6.

2. A method for forming a phosphate coating on the surface of a metal of the class consisting of steel, zinc and aluminum, which comprises the steps of contacting the surface of the metal with an aqueous acidic solution consisting essentially of phosphoric acid, water and a substituted amine phosphate having the structure:

wherein R1 and R2 are each a member of the class consisting of hydrogen and hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl,

aryl and cycloalkyl, and R3 is a member of the class consisting of hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, said phosphate yielding at least about .0l% of a phosphate ion in the solution, the pH of the solution being between 3 and 6, whereby the said solution chemically reacts with the surface to form a phosphate coating thereon, thereafter contacting the coated surface with an aqueous acidic phosphate solution consisting essentially of a phosphate of the class of coating phosphates to thereby form a second phosphate coating on the said surface, and thereafter rinsing the coated surface with a hexavalent chromium solution.

3. A composition for coating a metal of the class consisting of mild steel, zinc and aluminum comprising an aqueous acidic solution consisting essentially of phosphoric acid, water and a substituted amine phosphate having the structure:

R1 -NRa wherein R1 and R2 may each be a member of the class consisting of hydrogen and hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl, and cycloalkyl, and R3 may be a member of the class consisting of hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, the pH of the solution being between about 3 and 6.

4. A composition for coating a metal of the class consisting of mild steel, zinc and aluminum comprising an aqueous acidic solution consisting essentially of phosphoric acid, water, a small amount of an oxidizing agent, and a substituted amine phosphate having the structure:

wherein R1 and R2 may each be a member of the class consisting of hydrogen and hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, and R3 may be a member of the class consisting of hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, the pH of the solution being between about 3 and 6.

5. A method for cleaning and coating a metal of the class consisting of mild steel, zinc and aluminum which comprises the step of contacting the surface of the metal with an aqueous, acidic solution consisting essentially of phosphoric acid, water and a substituted amine phosphate have the structure:

wherein R1 and R2 may each be a member of the class consisting of hydrogen and hydrocarbon radicals having less than 7 carbon atoms and of the group consist-- ing of alkyl, aryl and cycloalkyl, and R3 may be a member of the class consisting of hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, and an oxidizing agent in an amount suificient to result in an increase in coating weight, said solution having a pH between about 3 and 6.

6. A method for cleaning and coating a metal of the class consisting of mild steel, zinc and aluminum which comprises the step of contacting the surface of the metal with an aqueous, acidic solution consisting essentially of phosphoric acid, water and a substituted amine phosphate having the structure:

. :7 wherein ;R1 and R2 may each be a member of 'tl1e:.class consisting of hydrogen .and hydrocarbon radicals having less ithan 7 carbon atoms and of the group consisting of allcyl, aryl and .cycloalkyl, and R3 maybe-a member of the class consisting of hydrocarbon radicals havingiless than .7 carbon atoms and ot'the group consisting of :alkyl, aryl and cycloa'lkyl, and :an oxidizing agent in an amount su'lficient to result in an increase in coating weight, said solution having a pH between about 3 and -'6, and thereafter rinsing the coated surface with a hexavalent chromium solution.

7. A method forcleaning and coating a metal of the class consisting of=mild steel, zinc and aluminum which comprises the step of contacting the surface of the metal with an acidic aqueous solution consisting essentially of phosphoric acid, water and a substituted amine-phosphate having the structure:

2n, N R2 s wherein R1 and R2 are each amember of the class consisting of hydrogen and hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and 'cycloalkyl, and R3 is a member of the class consisting of hydrocarbon radicals having less than '7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, and wherein the foregoing hydrocarbon radicals contain atleast 'one substituent selected from the-group consisting of hydroxyl, bromo,.chloro, sulfo and nitro, the pH of the solution'being between 3 and 6.

'8. A composition for coating a metal of the class consisting of mild steel, zinc and aluminum comprising an aqueous acidic solution consisting essentially of phosphoric acid, watertand a substituted amine phosphate having the structure:

R1 'N -Rg Rs wherein .R1 and R2 are each a member of the class consisting of hydrogen and hydrocarbon radicals having less than '7 carbon atoms and .of the group consisting of alkyl, aryl'and cycloalkyl, and R3 is a member of the class consisting of hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and .cycloalkyl, and wherein the foregoing hydrocarbon radicals contain at least one substituent selected from the group consisting of hydroxyl, bromo, chloro,, suite and nitro, the pH of the' solution being between about 3 and 6.

9. A composition for coating a metal of the class consisting of mild steel, zinc and aluminum comprising an aqueous acidic solution consisting essentially of a phosphoric acid, water, a small amount of an oxidizing agent, and a substituted amine phosphate having the structure:

' :8 carbon :atoms andof the group consisting .of alkyl,.aryl, and cycloalk-yl, :and wherein the foregoing hydrocarbon zradicals contain :at .least one substituent selected from the group consisting of hydroxyl, bromo, chloro, sulfo and mitro, the pH ;of :the solution being between about 3 and-6.

1-10. Annetbod :for cleaning :and coating :a metal'ofthe tclass consisting 10f mild steel, zinc and aluminum which comprises :the step .of contacting the surface of the metal .with an aqueous, :acidic :solution consisting essentially 0f phosphoric :acid, ':water .and a substituted amine phos- :phate :having .the structure: I

' 7 R1 .N/-R2 \R, wherein R1 and R2 are eacharmember of the class consisting of hy-drogenand hydrocarbon radicals having less than 7 carbon atoms and of the group consisting of alkyl, aryl and cycloalkyl, and R3 is a member of the class consisting of hydrocarbon radicals having less than 7 carbon atoms andof the group consisting of alkyl, aryl .and cycloalkyl, and wherein the foregoing hydrocarbon radicals contain at least one substituent selected from the'group consisting of hydroxyl, bromo, chloro, sulfo and nitro, and an oxidizing agent in an amount sufficient to result in an increase in coating weight, said solution having a pH between about 3 and 6.

'11. A method forcleaning andcoating a metal of the class .consistingotmild .steel, zinc and aluminum which comprises the step .ofvcontacting the surface'of the metal with an aqueous, acidic solution consisting essentially of phosphoric acid, .water and a substituted amine phosphate having the :structure: 7

R1 ar \R.

wherein R1 and R2 are :each a member of :the class con sisting of hydrogen and hydrocarbonradicals having less than 7 :carbon atoms and of Ithe group consisting of alkyl, arylsand cycloalk-yl, and R3 is a member of'the class consisting of hydrocarbon radicals having less than 7 carbonatoms and 'of the group consisting of alkyl, aryl and cycloalkyl, and wherein the foregoing hydrocarbon radicals contain at least one substituent selected'from the group consisting of hydroxyl, bromo, chloro, sulfo' and nitro, and an oxidizing agent in an amount sufiicient to result in an increase in coating weight, said solution.

References Cited in the file of this patent UNITED STATES PATENTS :1,428;0:8.7 .Grave'll Sept. 5,

1,805,982 xTGravell May 19, 1931 2,268,323 Martin Dec. .30, 1941 2,314,887 Lodeesen Mar. 30, 1943 2,318,606 Goebel' May 11, 1943 2,416,754 :Boggsetaal Mar. 4, 1947' 2,456,947 :lemstedt Dec. 21, 1948 2,479,423 :Snyder Aug. I6, 1949 2,552,874 Snyd'er et al. May 15, 1951 2,609- 3fl8 Gibson Sept. 2, 1952 

1. A METHOD FOR CLEANING AND PRODUCING A METALLIC PHOSPHATE COATING ON A METAL OF THE CLASS CONSISTING OF MILD STEEL, ZINC AND ALUMINUM WHICH COMPRISES THE STEP OF CONTACTING THE SURFACE OF THE METAL WITH AN ACIDIC AQUEOUS SOLUTION ESSENTIALLY OF PHOSPHORIC ACID, WATER AND A SUBSTITUTED AMINE PHOSPHATE HAVING THE STRUCTURE: 